setting gain on qhy168c

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emmett51
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Joined: Fri Sep 24, 2021 6:57 pm

setting gain on qhy168c

#1

Post by emmett51 »

I just read Robin's post about Sharpcap's update for the QHY168c adding a digital gain adjustment bar and gain adjustments bars for the red/green/blue channels.
Can someone explain what these do and how to set the gain using the four bars?
Just experimenting suggests that the color bars affect white balance. Do they also affect the camera gain? If so, what does the digital gain do? Which controls are actually affecting the raw data vs. the display vs. the histogram?

A second question about the camera: the QHY168c has a 14bit adc but scales this to 16bits unless the option turned off. I am having problems with color in post processing with Startools software which generally requires unstretched data. Could the scaling be the problem?

My objective is long exposures of deep sky objects. In the past, I have used dslr's and 16bit ccd cameras with gain and offset set by the manufacturer. The cmos camera is a mystery so far. I have read the material on the QHY site but it is confusing. All help appreciated.
ER
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Re: setting gain on qhy168c

#2

Post by admin »

Hi,

CMOS cameras typically have variable gain, changes to the gain affect the read noise and e/ADU (and hence FWD). Depending on the tracking capabilities of your mount how dark your sky background is, it may be a good idea to increase the gain from minimum to take advantage of the reduced read noise that brings (reduced read noise brings down required sub-exposure length... half the read noise and the required sub length drops by a factor of four).

In the case of the QH168 (IMX071 sensor), the camera has separately adjustable gains for the three colour channels. I have no idea whether this is actually very useful, but it's just how QHY have set up the SDK for this camera. I would start off by setting all to the same value. The other gain control is a digital gain - this should be left at the default value so that no digital gain is applied.

If you are saving to FITS, you can turn off the stretching (see https://docs.sharpcap.co.uk/4.0/#Saving%20Options , although this has now moved to a separate saving tab in the settings).

Hope this helps,

Robin
emmett51
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Joined: Fri Sep 24, 2021 6:57 pm

Re: setting gain on qhy168c

#3

Post by emmett51 »

Thanks Robin,
Can you help with another problem I'm having?

I'm trying to follow the instructions in the QHY169c user manual to set the gain and offset.
The instructions say to use EZCAP and set the gain and exposure to zero then use the noise analyze function to get an average value between 500 and 3000. My version of EZCAP doesn't allow zeros for gain or exposure. The noise analyze function does not give an average so I'm using the average of the max/min that is shown. When I use the lowest settings for gain and exposure and increase the offset to 100, the average still is well below 500.

I removed the lens cover and tried to set the gain using EZCAP and the noise analyze function. The local average values (avg max/min ?) all exceed 65k unless I use the lowest gain and lower the offset to 9. Even then, the max is over 65k.

This result doesn't seem right to me. I have tried to do something similar in SharpCap and Nebulosity. I can't seem to get any separation of data from the left edge of the histogram without setting the offset to maximum. When I try to capture images, that separation disappears and the data is at the left edge. Any faint detail in the image disappears. I'm starting to suspect my camera may need servicing.

In your last reply, you mentioned default values for gain/offset for the 168c. Could you tell me where I might find them?
Thanks again for your help.
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Re: setting gain on qhy168c

#4

Post by admin »

Hi,

I was rather hoping that you would already know the default values for things like the gain... QHY cameras tend to be a bit tricky in this area because the defaults vary from model to model :( As you have noticed the same applies to the gain values - some cameras have gain starting at zero, measured in dB or 0.1 dB. Others have gains starting at 1, where the units seem arbitrary. It all makes it very hard to recommend settings for QHY cameras...

As to the EZCAP procedure, I am afraid I'm not familiar with it.

Have you tried using the SharpCap smart histogram 'Brain' function? That should calculate gain, exposure and offset settings appropriate for your conditions based on measurements of your sensor noise and control characteristics. SC4.0 ships with default sensor data measured previously for the QHY168C, and should therefore understand what various gain values mean in terms of actual gain amount and how the various offset levels adjust the black point of the image (it will choose an offset to separate the histogram from the left hand side). The only thing I am not quite sure about is if it will correctly set all three colour gain sliders to the recommended gain level (I have no camera with individual colour gains to test this with).

https://docs.sharpcap.co.uk/4.0/#The%20 ... n%20Window

cheers,

Robin
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